Whereas various packaging materials for packaging various goods, such as beverages, foods, pharmaceuticals, chemicals, items for daily use and sundries, have been developed and proposed, soft plastic packaging bags are often used due to their handling ease, lighter weight, convenience and reduced volume of waste matter. These soft packaging materials are required to have various properties including a barrier performance against oxygen gas, water vapor and the like, flexibility, shock resistance, wear resistance, pin hole-proofness, pierce resistance, transparency, heat resistance, low-heat sealing performance, quality holding capability, printability, opening ease, packaging performance and so forth. Especially where the content is fluid, or to be preserved at low temperature, the pin hole-proofness of the packaging bag is a fundamental leak prevention of the content, and packing materials using many different film materials or configurations have been proposed.
Although a single layer film is often used as the material of packaging bags, it is preferable because of the need to satisfy the property requirements stated above to use a laminated film provided with greater film strength, heat sealing performance and functions including gas-barrier performance. Regarding the configuration of the laminated film, as the base film for securing a mechanical strength, a high-strength biaxially oriented polyester film, a biaxially oriented nylon film excelling in low-temperature fatigue or anti-shock performance such as shock resistance or the like is used either by itself or in combination with another; as regards the permitting heat sealing, generally a polyethylene film, above all a linear low-density polyethylene film excelling in many properties including heat sealing strength, stress crack resistance, shock resistance and low-temperature performance, is preferably used.
Now, against the generation of pin holes by the repetition of bending and abrasion of a packaging bag mainly by vibration in the transportation process, a pin hole-resistant packaging material of a configuration having a partially unstuck region between the base film layer and the sealant layer is proposed. Thus, there is disclosed a packaging material characterized in that a polyolefin resin film that can serve as the sealant layer and a gas barrier layer having a gas barrier property are stacked one over the other with an adhesive layer in which a material adhesive to these two layers and a material having not adhesive thereto are mixed and dispersed in any desired proportion in-between, and the above described material adhesive to the layers constitutes 50% or more of the surface of the object of adhesion (refer to Patent Document 1).
Also, as another means of solution, there is disclosed a technique by which pin hole-proofness and fall resistance are improved by layering two or more laminated films into a multi-layered bag to have an outer bag intervene between the innermost bag which directly packages liquid and the external container (cardboard), thereby preventing the innermost bag and the cardboard from being caused to directly rub each other by vibration while in transit or otherwise and, when the bag is bent, preventing the bending stress working on the laminated film from increasing. More specifically, it is a packaging material characterized by being composed of a two or more-layered film in which at least one kind of film selected from a group comprising biaxially oriented nylon films and biaxially oriented polyester films and a linear low-density polyethylene film are block-adhered to each other (refer to Patent Document 2).
Incidentally, pin holes are often generated by the combined effect of a number of different factors; formed and filled packaging bags are subject to the generation of pinholes, during the handling process or the distribution process until they are collected and packaged into a cardboard box, by the mutual rubbing of the cardboard box and the packaging bags caused by shearing destruction or vibration due to a drop impact or the bending fatigue of the packaging bags' own films. Against these points, past attempts have been made to seek for film configuration for enhancing the packaging bag's own pin hole-proofness.
However, as an inherent cause of occurrence of a pin hole, there has been found out a phenomenon in which the projection of a corner of a packaging bag pierces an adjacent packaging bag. Especially in a state of being filled with contents, the acutely angled portion formed by the bending of the boundary part between a lateral seal portion of a packaging bag and the content filling portion (hereinafter referred to as angular portion) is formed. And this angular portion invites mutual boring of pin holes between packaging bags by piercing or scratching adjacent packaging bags in the handling or transportation process.
For instance, FIG. 10A is a diagram showing the state in which a three-lateral sealed pillow packaging bag whose lateral seal portions 100A and 100B are formed by linear straight seals where it is made a pillow type packaged body 200 by filling the content filling portion 2 of that pillow packaging bag with liquid and sealing it. FIG. 10B is a diagram showing on an enlarge scale the upper left corner, which is one of the four corners of the pillow type packaged body 200 of FIG. 10A. It can be readily understood that an extremely acute angular portion 4 is formed at an end of the boundary between the lateral seal portions 100A or 100B and the content filling portion 2 as shown in FIG. 10B.
And when a plurality of such pillow type packaged bodies are packed into one cardboard box and, after being subjected to oscillation which simulates transportation, the cardboard box is opened, a pierced scar or a pin hole will be observed in an adjacent pillow type packaged body in the part where the angular portion meets. Similarly, in a collective packaged unit where an external bag is packed with small packaging bags in bulk, a major problem may arise. Such an acutely shaped angled portion is also apt to give rise to pin holes through rubbing with an external container (cardboard box).
It is difficult to cope with pin holes occurring from such causes merely by ameliorating the film material or the film configuration to improve the strength and other characteristics, and they often give rise to problems in protection or leakage of the content.
Now, there is disclosed an attempt against pin holes arising from friction with a cardboard box which is the external box, not pierced pin holes mentioned above, to expand the seal width of the shape of lateral seal portions while having it form a curve along the sealing side end portions (hereinafter referred to as R-seals) in a packaging bag the shape of whose lateral seal portions form a quadrilateral (refer to Patent Document 3). However, this provides no full solution as it involves such problems as susceptibility to angle formation due to a positional deviation in the curved portion, biting of the content into the seal portion due to the expansion of the area of the seal portion, sealing failure invited by an increase in the quantity of dirt, and a phenomenon of partial thinning of the thickness of the packaging material due to localized variation of the bearing at the time of sealing (hereinafter referred to as poly-slimming), further entailing difficult in machining the seal bar.
Further, though not intended to restrain pierced pin holes mentioned above, there is disclosed an attempt, for a pillow type food containing packaging bag for use with light food, such as snacks, to expand the sealing width in the central part while causing the shape of its lateral seal portions to form a curve (refer to Patent Document 4). However, this technique described in Patent Document 4 takes no heed of the function of the bag to be filled with and to package a fluid content, such as liquid or viscous fluid. As a result, folding due to the fall of the lateral seal portions may arise, still giving rise to angle formation at both side ends of lateral seals. Especially according to this technique described in Patent Document 4, as it is intended for use with light food, such as snacks, the lateral seal portions embody no consideration to prevent sealing failure caused by the intervention of the fluid content, and in this respect it is inadequate as a bag to be filled with and package a fluid content, therefore unable solve the problem of having to restrain pierced pin holes mentioned above.
[Patent Document 1] JP2004-148635A
[Patent Document 2] JP3515194B
[Patent Document 3] JP2000-185743A
[Patent Document 4] JP6-127556A
[Non-Patent Document 1] HIROSHI OSUGA, SHIN SHOKUHIN HOSO-YO FILM (Food Packaging Films, New Edition), published by NIPPON KITAKU HAMBAI KABUSHIKI KAISHA